The invention pertains to a procedure for the treatment of sewage by means of photosynthetically active algae and to a device for the conduction of the procedure. In particular, the invention pertains to a procedure and a device, which permits sewage treatment with extensive nutrient recycling by biomass production.
The use of algae mass and/or mixed algae cultures in different plant configurations for biomass production and/or sewage treatment, also in combination with biogas plants, is known already; see DE 196 11 885 C1 and DE 197 21 243 C2, for example. The biomass produced by means of this procedure can be processed for food products. Furthermore, diverse products, such as fine chemicals, vitamins, colouring agents, etcetera, can be extracted as well.
Within the framework of sewage treatment, the biomass accruing is usually treated as a residual material to be disposed of, which for example is disposed of by digestion processes, or which can be added as renewable raw materials to the biogas production in biogas plants.
Regardless of the intended use, the algae biomass has to be removed concentrated from the aqueous culture media in the previous culture technologies.
For various reasons, almost all large culture techniques so far are designed to cultivate floating algae; see DE 197 52 542 A1, for example. Algae can be referred to as floating when they neither swim on the surface nor settle under resting conditions. For this reason, they have to be concentrated and harvested by filtration or centrifugation from the aqueous solution in a rather complex process.
Apart from a number of other technological problems to be taken into account during the cultivation of free-floating algae—such as sufficient CO2 supply and/or prevention of an excessive O2 over-saturation as well as the handling of strongly varying light intensities and temperatures in the course of the day and year, especially in case of open systems—the much too expensive harvesting technique continues to be the main reason for the very sparse large-scale use of this technology to date.
Although a process has been described in EP 1 972 602 B1 already, with which freely floating as well as quickly settling algae populations can be bread in a targeted process.
On the one hand, however, this process has the drawback that on account of the open mode of operation under the climatic conditions given in Central and Northern Europe, where in winter the outside temperatures may drop below freezing and/or the daylight intensity in winter can be so low that no or hardly any net photosynthesis production is possible, the process cannot be operated all-year round, in particular also because any counter-measures to be taken, such as artificial illumination an/or heating, are uneconomic.
On the other hand, however, a research project funded by the DBU (ref. No. 25907) has revealed that a selective cultivation of the easy-to-harvest settling algae based on patent EP 1 972 602 B1 is possible only during very short hydraulic residence times of the water phase, otherwise this will lead to freely floating planktic micro-algae even in case of longer hydraulic residence times.